1. Field of the Invention
The present invention relates to an office machine such as a color photocopier, color printer, etc., having plural electrophotographic developers, and more particularly, to a contact shock dampening and development nip forming apparatus for developers which dampens contact shocks occurring when color developing rollers come in contact with the photosensitive drum, and maintains a thickness of the development nip.
2. Description of the Related Art
In general, an electrophotographic color image forming apparatus 10 (see FIG. 1), such as a color photocopier, color printer, etc., has a drum-shaped photosensitive body 11 continuously rotating in one direction due to a photosensitive body driving source (not shown). Adjacent to the circumference of the photosensitive body 11 is a first charger 12, a laser scanning unit (LSU) 20, and four contact development-type developers 31, 32, 33, and 34 accommodating yellow, magenta, cyan, and black developing agents. The apparatus 10 also includes a transfer roller 60, a discharge lamp 87, and a cleaning discharger 80, respectively in predetermined locations.
The first charger 12 is a scorotron charger and uniformly charges the photosensitive body 11, and the LSU 20 exposes the photosensitive body 11 to laser light in an axial direction to form a line.
Each of the developers 31, 32, 33, and 34 has a developing roller 13, a developing agent container 16, a developing agent supply roller 15, and a developing agent layer thickness regulation member 51 regulating the thickness of a developing agent attached on the developer 13. The developers 31, 32, 33 and 34 are each rotated by a developer driving source (not shown). The developing agent is supplied to the developing roller 13 through the developing agent supply roller 15 of the developing agent container 16, and regulated into a thin layer on the developing roller 13 by the developing agent layer thickness regulation member 51.
The respective developers 31, 32, 33, and 34 are reciprocated in a developer guide member (not shown), and move against release springs 74 toward the photosensitive body 11 by cams 35, 36, 37, and 38 fixed to rotation shafts 56. The rotation of the rotation shafts 56 is regulated by electronic clutches (not shown).
Further, the developing roller 13 is applied with a developing bias voltage upon development. The developing bias voltage has the same polarity as the photosensitive body 11 if the development involves negative-positive reversal.
The transfer roller 60 electrostatically transfers onto a sheet of recording paper a color image formed on the photosensitive body 11, and the cleaning discharger 80 removes developing agents remaining on the photosensitive body 11.
In the operation of the image forming apparatus 10, first, when a printing command is input, the photosensitive body 11 continuously rotates due to the photosensitive driving source. Thereby, the surface of the photosensitive body 11 is uniformly charged by the first charger 12. When a charged region is reached, for example, on a developing location d of the yellow developer 31 for a first color image, the electronic clutch of the yellow developer 31 is electrically engaged, so that the yellow developer 31 moves toward the photosensitive body 11 by the eccentric cam 35. The developer 31 is thus set in a developing state.
Next, the surface of the photosensitive body 11 is exposed to light from the LSU 20 to form a yellow electrostatic latent image thereon, and at the developing location d a yellow image is continuously developed from the leading end to the rear end of the image.
After the yellow image is completely formed and the rear end of the image passes through the developing location d, the eccentric cam 35 rotates, and thereby the yellow developer 31 is separated from the photosensitive body 11.
Thereafter, when the leading end of the image is reached, for example, on the developing location e of the magenta developer 32 for a second color image, the electronic clutch of the magenta developer 32 is electrically engaged, so that the magenta developer 32 is set in the developing state by the eccentric cam 36.
At this time, the yellow image formed on the photosensitive body 11 passes through the transfer roller 60, the discharge lamp 87, and the cleaner discharger 80, which are maintained in a non-operation state, and is again positioned underneath the first charger 12. In particular, the transfer roller 60 and the cleaning discharger 80 are generally set in a state of non-contact with the photosensitive body 11. However, contact is necessary to prevent fading of the image.
Underneath the first charger 12, the photosensitive body 11 having the yellow image is evenly charged by the first charger 12 again, and then a latent image corresponding to the magenta color is superimposed on the yellow image by exposing the photosensitive body 11 to light from the LSU 20, and developed on the magenta developing location e. After the magenta image is completely formed and the rear end of the image passes through the magenta developing location e, the eccentric cam 36 rotates, so that the magenta developer 32 is separated from the photosensitive body 11.
Next, when the rear end of the image is reached, for example, on a developing location f of the cyan developer 33 for a third color image to be formed, the electronic clutch of the cyan developer 33 is electrically engaged so that the cyan developer 33 is set in the developing state by the eccentric cam 37.
At this time, the superimposed image of yellow and magenta have passed through the transfer roller 60, the discharge lamp 87, and the cleaner discharger 80 is again located underneath the first charger 12. Thus, the photosensitive body 11 is evenly charged by the first charger 12. Further, the image of yellow and magenta is superimposed with the cyan image by exposure to light from the LSU 20. The cyan image is developed on the cyan developing location f. After the cyan image is completely formed and the rear end of the image passes through the cyan developing location f, the eccentric cam 37 rotates so that the cyan developer 33 is separated from the photosensitive body 11.
Next, a black image is superimposed and formed in the same manner, so that all of the images are completely formed. The color image formed on the photosensitive body 11 is transferred onto a sheet of recording paper P synchronously delivered from a paper supply unit (not shown) by the transfer roller 60.
After the transfer, the photosensitive body 11 is discharged by the discharge lamp 87, the developing agent remaining on the surface of the photosensitive body 11 is removed by a rotation brush 81 of the cleaning discharger 80, so that the photosensitive body 11 returns to the initial state. At this time, the paper P on which the image is transferred is delivered to a fixing unit (not shown) to be fixed, and is externally discharged.
As described above, the conventional image forming apparatus 10 has a structure in which the developing rollers 13 of the four color developers 31, 32, 33, and 34 come in contact with the photosensitive body 11 for development. Thus, the photosensitive body 11 performs four complete developing processes, one for every color. The developing rollers 13 of the respective developers 31, 32, 33, and 34 come in or out of contact with the photosensitive body 11 due to pressure from the eccentric cams 35, 36, 37, and 38 to be in a developing or a waiting position.
However, the conventional photosensitive body 11 is generally constructed with a cylindrical member having a high hardness, such as aluminum, having a photosensitive layer coated on the outer circumference thereof. Further, the developing rollers 13 are also constructed with cylindrical members of high hardness, such as aluminum or resin. Accordingly, the contact shocks occurring when the photosensitive body 11 performs the developing processes in contact with the developing rollers 13 of the developers 31, 32, 33, and 34 are directly transferred to the entire photosensitive body 11. Thus, the quality of images developed on the photosensitive body 11 becomes unstable.
Further, a pressure variation occurring when the developing rollers 13 come in or out of contact with the photosensitive body 11 due to the eccentric cams 35, 36, 37, and 38 for switching to the next developers 31, 32, 33, 34 upon developing is directly transferred to the photosensitive body 11. This occurs during different operations, for example, charging or exposing to light. Furthermore, damage to the photosensitive layer of the photosensitive body 11 may result, thereby causing a problem in which the quality of images developed on the photosensitive body 11 is deteriorated. Also, the life span of the photosensitive body 11 is shortened.